Update .abi-compliance-history file.

Update .abi-compliance-history file.

As foretold by commit a72f7d97be, this commit moves the baseline
point for ABI compatibility for v18 to commit c8af5019be.  While at
it, add some more commentary and adjust the format of the entries
to improve both human and machine readability.  There's a good
chance we'll add an .abi-compliance-history file to the other
back-branches, but for now this effort is limited to v18.

Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Reviewed-by: Peter Geoghegan <pg@bowt.ie>
Reviewed-by: David E. Wheeler <david@justatheory.com>
Reviewed-by: Mankirat Singh <mankiratsingh1315@gmail.com>
Discussion: https://postgr.es/m/aPJ03E2itovDBcKX%40nathan
Backpatch-through: 18 only